The present invention relates to a method of manufacturing an internal gear, an internal gear structure and a reduction mechanism unit having an internal gear structure. In particularly, the present invention relates to a method of manufacturing a thin and light-weight internal gear which is provided on a reduction mechanism unit of a starter for use in an automobile. The internal gear structure of the present invention can apply an internal gear structure with which a several planet gears intermesh.
In a method of manufacturing an internal gear and an internal gear structure according to the present invention, a material for forming the internal gear employs a simple shape sheet metal material and this sheet metal material is processed by a press working. This sheet metal material is formed into a hollow cylindric metal body that each portion has an appropriate thickness as an intermediate workpiece and this intermediate workpiece is formed as a completed product of the internal gear structure using a roll forming method.
The roll forming method comprises the steps of: (1) fitting an inner peripheral surface of a cylindric portion of a hollow cylindric metal body into an outer peripheral surface of a mandrel having a tooth profile part; (2) clamping the hollow cylindric metal body at both axial ends thereof to hold the hollow cylindric metal body; (3) rotating the metal hollow cylindric body together with the mandrel, a drive shaft and a driven shaft those are fixed by pressing one other; (4) pressing a roller radically on an outer peripheral surface of the cylindric portion of the hollow cylindric metal body in a direction perpendicular to an axial of the mandrel while rotating the hollow cylindric metal body together with the mandrel; whereby the hollow cylindric metal body is plastically deformed, thereby an internal tooth profile is formed on the inner peripheral surface of the cylindric portion of the hollow cylindric metal body.
As a conventional technique of a reduction mechanism structure having an internal gear structure, various techniques have been developed. Within these techniques, the typical examples of the conventional reduction mechanism structure techniques having the internal gear structure are as follows.
(1) A reduction mechanism structure having an internal gear structure in which a sun gear is provided on a drive shaft and between this sun gear and the internal gear structure several planet gears are disposed to be in intermesh with the sun gear and the internal gear structure.
(2) A reduction mechanism structure having an internal gear structure in which a cum or a crank is provided on a driven shaft, and by the cum or the crank the planet gears intermeshed with the internal gear structure are supported rotatively at each center portion.
(3) A reduction mechanism structure having an internal gear structure in which an external gear is provided on a drive shaft, and this external gear is meshed directly with the internal gear structure so that this internal gear can be rotated by the drive shaft.
Besides, as an example of the conventional method for manufacturing an internal gear structure, there is a non-cutting method shown in U.S. Pat. No. 4,884,427. It shows that a helical internal gear structure is manufactured by a following method. Namely, a hollow cylindric metal body for forming the helical internal gear structure is formed by the roll forming apparatus including a mandrel having a tooth profile part at an outer peripheral surface and a roller.
The helical internal gear structure manufactured by the above stated conventional non-cutting method has the merit in that an accuracy of the tooth profile can be as high as a tooth profile by a conventional cutting method with the reduced number of machining steps.
However, since in generally most of the shape of the hollow cylindric metal body is formed by the forging working, that known method has still following various problems to be improved.
(1) It is difficult to form a thin and light-weight internal gear structure having sufficient strength.
(2) It is difficult to install a boss in the internal gear in order to fix it to a housing or a bracket of a motor etc.
(3) The cost of the material and the processing, including the surface treatment and the thermal treatment to manufacture the internal gear structure is apt to be high.
A method and apparatus for splining clutch hubs is disclosed in, for example U.S. Pat. No. 4,596,127. In this prior art, in pressure forming splines or teeth in an axially extending sleeve of a clutch hub (cup-shaped power transmission member), a pair of special tooth forming racks adapted to intermesh with a toothed mandrel with the sleeve therebetween and a pair of special support racks adapted to contact an oil seal surface are used.
However, in U.S. Pat. No. 4,596,127, since the internal gear is formed by a flat rolling, there is no restriction mechanism which restricts the flow of the metal material toward an axial direction. Though splines of the clutch hub formed by this method, since a regular amount of the metal material does not flow into a portion to be formed the tooth profile part, the internal gear structure having a precise tooth profile part which is enable to intermesh with a several planet gears can not formed by this method.
Besides, in a case that an internal gear structure is formed by a sheet metal material, in generally it can obtain the internal gear structure having a thin thickness structure. When this internal gear structure is adapted to intermesh with several planet gears, it is important to form a strong and rigid internal gear structure so as to intermesh with the planet gears and such a sleeve shape internal gear structure may be easily deformed and so that the above stated internal gear structure can not intermesh with the planet gears.